lazyeval 0.2.0

Formula-based lazy evaluation

Lazyeval has a new system for lazy-eval based on formulas, described in depth in the new lazyeval vignette. This system is still a little experimental - it hasn't seen much use outside of the vignette, so it certainly may change a little in the future. However, long-term goal is to use these tools across all of my packages (ggplot2, tidyr, dplyr, etc), and I am fairly confident that this is a robust system that won't need major changes.

There are three key components:

• f_eval() evaluates a formula in the environment where it was defined.
  If supplied, values are first looked for in an optional data argument.
  Pronouns .data and .env can be used to resolve ambiguity in this case.
  (#43). Longer forms f_eval_rhs() and f_eval_lhs() emphasise the side
  of the formula that you want to evaluate (#64).
• f_interp() provides a full quasiquoting system using uq() for unquote
  and uqs() for unquote-splice (#36).
• f_capture() and dots_capture() make it easy to turn promises
  and ... into explicit formulas. These should be used sparingly, as
  generally lazy-eval is preferred to non-standard eval.
• For functions that work with ..., f_list() and as_f_list() make it
  possible to use the evaluated LHS of a formula to name the elements of a
  list (#59).

The core components are accompanied by a number of helper functions:

• Identify a formula with is_formula().
• Create a formula from a quoted call and an environment with f_new().
• "Unwrap" a formula removing one level from the stack of parent environments
  with f_unwrap().
• Get or set either side of a formula with f_rhs() or f_lhs(), and
  the environment with f_env().
• Convert to text/label with f_text() and f_label().

I've also added expr_find(), expr_text() and expr_label() explicitly to find the expression associated with a function argument, and label it for output (#58). This is one of the primary uses cases for NSE. expr_env() is a similar helper that returns the environment associated with a promise (#67).